The present invention relates to rectifier circuits, and more particularly to rectifier circuits that rectify three phase AC power sources without a neutral connection.
Rectifier circuits are commonly used for converting an alternating current (AC) signal into a direct current (DC) signal. Applications that require either DC power AC power at a different frequency initially require the 50-60Hz three phase AC power to be rectified. The rectified DC power can then be used or processed using power conversion modules.
Some applications require DC power at a higher or lower level than the rectified DC voltage. In this situation, a power conversion module converts the DC power to the desired higher or lower DC level. When AC power at a different frequency or voltage is desired, the rectified DC power is inverted by a power conversion module to AC at the desired voltage or frequency.
In some situations, it is desirable to run a DC-AC power conversion module without using a regulated DC power supply. Certain types of power conversion modules (especially 1MHz and up) become significantly less efficient as their DC supply voltage is increased.
There are many applications for power conversion modules that are supplied by 400VAC (common in foreign countries) or 480VAC (common in the United States) mains. These applications include RF amplifiers and RF generators. The standard practice for high frequency power conversion modules is to connect two or more lower voltage power conversion modules in series. However, if one of the series connected modules fails during operation, the whole system fails. It is also difficult to share the rectified DC voltage input evenly between the series connected power conversion modules.
When the AC supply is three phase, three AC signals and a ground and/or neutral are typically provided. When the neutral is available, the voltages that are delivered to the circuit can be phase-to-phase or phase-to-neutral. In many facilities, however, the neutral connection is not available. When no neutral connection is available, the voltages that are delivered to the circuit can be only phase-to-phase. The phase-to-neutral voltages are typically lower than the phase-to-phase voltages by a factor that is equal to 3. The lower voltages allow higher efficiency in DC-AC high frequency applications and a phase-to-neutral connection would be utilized if availability of the neutral connection was guaranteed. Also, there are current restrictions for the neutral wires, and any application which utilizes a neutral connection provided by their facility is subject to those restrictions.
A rectifier circuit according to the present invention powers three identical power conversion modules using a three phase AC input without a neutral connection from a power line. A first bridge rectifier is connected to a first phase of the three phase AC input and produces a first rectified waveform. A second bridge rectifier is connected to a second phase of the three phase AC input and produces a second rectified waveform. A third bridge rectifier is connected to a third phase of the three phase AC input and produces a third rectified waveform. A connection between the first, second, and third bridge rectifiers forms a virtual neutral.
In other features of the invention, capacitors are connected across the bridge rectifies to filter the rectified waveforms and to create a DC rail voltage. The three DC rail voltages are of equal magnitude. The three DC rail voltages feed three power conversion modules. Two of the three power conversion modules remain powered at a decreased voltage level when the remaining power conversion module fails.
In yet other features, inductors are connected between the bridge rectifiers and the capacitors to further smooth the DC rail voltages and increase the power factor. Alternately, inductors are connected between the phases of the three phase AC input and the bridge rectifiers to smooth the DC rail voltage and to increase the power factor.
In still other features of the invention, the bridge rectifiers include first, second, third, and fourth diodes, each with an anode and a cathode. The anode of the first diode is connected to the cathode of the second diode. The cathode of the first diode is connected to the cathode of the third diode. The anode of the second diode is connected to the anode of the fourth diode. The anode of the third diode is connected to the cathode of the fourth diode. The anode of the first diode of the first bridge rectifier is connected to the first phase of the three phase AC input. The anode of the first diode of the second bridge rectifier is connected to the second phase of the three phase AC input. The anode of the first diode of the third bridge rectifier is connected to the third phase of the three phase AC input. The virtual neutral includes a conductor that connects the anode of the third diode of the first bridge rectifier, the anode of the third diode of the second bridge rectifier, and the anode of the third diode of the third bridge rectifier.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.